A wide variety of interferometric imaging techniques have been developed to provide high resolution structural information in a wide range of applications. Optical Coherence Tomography (OCT) is a technique for performing high-resolution cross-sectional imaging that can provide images of samples including tissue structure on the micron scale in situ and in real time (see for example Huang et al. “Optical Coherence Tomography” Science 254 (5035): 1178 1991). OCT is an interferometric imaging method that determines the scattering profile of a sample along the OCT beam by detecting light reflected from a sample combined with a reference beam. Each scattering profile in the depth direction (z) is called an axial scan, or A-scan. Cross-sectional images (B-scans), and by extension 3D volumes, are built up from many A-scans, with the OCT beam moved to a set of transverse (x and y) locations on the sample.
Many variants of OCT have been developed where different combinations of light sources, scanning configurations, and detection schemes are employed. In time domain OCT (TD-OCT), the path length between light returning from the sample and reference light is translated longitudinally in time to recover the depth information in the sample. In frequency domain or Fourier domain OCT (FD-OCT), the broadband interference between reflected sample light and reference light is acquired in the spectral domain and a Fourier transform is used to recover the depth information. The sensitivity advantage of frequency-domain optical coherence tomography (OCT) over time-domain OCT is well established (see for example Choma et al. “Sensitivity advantage of swept source and Fourier domain optical coherence tomography,” Opt. Express 11, 2183-2189, 2003 and Leitgeb et al. “Performance of Fourier domain vs. time domain optical coherence tomography,” Opt. Express 11, 889-894, 2003).
There are two common approaches to FD-OCT. One is spectral domain OCT (SD-OCT) where the interfering light is spectrally dispersed prior to detection and the full depth information can be recovered from a single exposure. The second is swept-source OCT (SS-OCT) where the source is swept over a range of frequencies and detected over time, therefore encoding the spectral information in the time dimension. In traditional point scanning or flying spot techniques, a single point of light is scanned across the sample. In parallel techniques, a series of spots, a line of light (line-field), or a two-dimensional array of light (full-field) are directed to the sample. The resulting reflected light is combined with reference light and detected. Parallel techniques can be accomplished in TD-OCT, SD-OCT or SS-OCT configurations.
Several groups have reported on different line-field FD-OCT configurations (see for example Zuluaga et al. “Spatially resolved spectral interferometry for determination of subsurface structure”, Optics Letters 24, 519-521, 1999; Grajciar et al. “Parallel Fourier domain optical coherence tomography for in vivo measurement of the human eye”, Optics Express 13, 1131, 2005; Nakamura et al. “High-speed three-dimensional human retinal imaging by line-field spectral domain optical coherence tomography” Optics Express 15(12), 7103-7116, 2007Mujat et al. “Swept-source parallel OCT” Proceedings of SPIE 7168, 71681E, 2009; Lee et al. “Line-field optical coherence tomography using frequency-sweeping source” IEEE Journal of Selected Topics in Quantum Electronics 14(1), 50-55, 2008).
The related fields of optical diffraction tomography, holoscopy, digital interference holography, holographic OCT, and interferometric synthetic aperture microscopy (see for example Hillman et al. “Holoscopy—holographic optical coherence tomography: Optics Letters 36(13), 2390-2392, 2011; U.S. Pat. No. 7,602,501; and Kim M K “Tomographic three-dimensional imaging of a biological specimen using wavelength-scanning digital interference holography” Optics Express 7(9) 305-310, 2000) are also interferometric imaging techniques that can be accomplished in parallel and in line-field scanning configurations (see for example U.S. patent application Ser. No. 13/745,632 hereby incorporated by reference).
In a point scanning interferometric system, one typically uses single mode fibers to connect the source to the interferometer, as well as to connect the interferometer to the detection unit. These single mode fibers serve at the same time as spatial filters, which only allow the collection of a single transverse mode. Therefore all the detected light coherently contributes to the interference signal. In line-field systems, the sample is however in contrast to a point scanning OCT system illuminated by a line of light, instead of a focused spot. Through back scattering in the sample, the line of light is projected onto the detector, the imaging relations between the sample and the linear photodiode array unfortunately do not permit the light to be guided by a single mode fiber from the interferometer to the detection unit. The single mode spatial filter known from point scanning systems is therefore missing. In order to implement a spatial filter in free space optics, previously published line-field OCT systems employed a slit as a spatial filter. A slit can however only provide limited transverse mode selection, still allowing multiple transverse modes to propagate to the detector as it does not discriminate based on the angle of incidence. Therefore it is very difficult to make all the light on the detector contribute coherently to the interference fringe signal. Instead the extra modes create an incoherent background only adding to the noise but not the signal. Also a large amount of multiply scattered light may pass through the slit, which may result in a further reduction of image quality.